The wireless local-area network (WLAN) technology known as “Wi-Fi” has been standardized by IEEE in the 802.11 series of specifications (i.e., as “IEEE Standard for Information technology—Telecommunications and information exchange between systems. Local and metropolitan area networks—Specific requirements. Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications”). As currently specified, Wi-Fi systems are primarily operated in the 2.4 GHz or 5 GHz bands.
The IEEE 802.11 specifications regulate the functions and operations of the Wi-Fi access points or wireless terminals, collectively known as “stations” or “STA,” in the IEEE 802.11, including the physical layer protocols, Medium Access Control (MAC) layer protocols, and other aspects needed to secure compatibility and inter-operability between access points and portable terminals. Because Wi-Fi is generally operated in unlicensed bands, communication over Wi-Fi may be subject to interference sources from any number of both known and unknown devices. Wi-Fi is commonly used as wireless extensions to fixed broadband access, e.g., in domestic environments and in so-called hotspots, like airports, train stations and restaurants.
Recently, Wi-Fi has been subject to increased interest from cellular network operators, who are studying the possibility of using Wi-Fi for purposes beyond its conventional role as an extension to fixed broadband access. These operators are responding to the ever-increasing market demands for wireless bandwidth, and are interested in using Wi-Fi technology as an extension of, or alternative to, cellular radio access network technologies. Cellular operators that are currently serving mobile users with, for example, any of the technologies standardized by the 3rd-Generation Partnership Project (3GPP), including the radio-access technologies known as Long-Term Evolution (LTE), Universal Mobile Telecommunications System (UMTS)/Wideband Code-Division Multiple Access (WCDMA), High Speed Packet Access (HSPA) and Global System for Mobile Communications (GSM), see Wi-Fi as a wireless technology that can provide good additional support for users in their regular cellular networks.
As used herein, the term “operator-controlled Wi-Fi” indicates a Wi-Fi deployment that on some level is integrated with a cellular network operator's existing network, where the operator's radio access network(s) and one or more Wi-Fi wireless access points may even be connected to the same core network (CN) and provide the same or overlapping services. Currently, several standardization organizations are intensely active in the area of operator-controlled Wi-Fi. In 3GPP, for example, activities to connect Wi-Fi access points to the 3GPP-specified core network are being pursued. In the Wi-Fi alliance (WFA), activities related to certification of Wi-Fi products are undertaken, which to some extent is also driven from the need to make Wi-Fi a viable wireless technology for cellular operators to support high bandwidth offerings in their networks. In these standardization efforts, the term “Wi-Fi offload” is commonly used and indicates that cellular network operators seek means to offload traffic from their cellular networks to Wi-Fi, e.g., during peak-traffic-hours and in situations when the cellular network needs to be off-loaded for one reason or another, e.g., to provide a requested quality-of-service, to maximize bandwidth, or simply for improved coverage.
Traffic Offloading Using Wi-Fi
As noted above, using Wi-Fi/WLAN (the two terms are used interchangeably throughout this application) to offload traffic from the mobile networks is becoming more and more interesting from both the operator's and end user's points of view. Some of the reasons for this tendency are:                Additional frequency: by using Wi-Fi, operators can access an additional 85 MHz of radio bandwidth in the 2.4 GHz band and another (close to) 500 MHz in the 5 GHz band.        Cost: From the operator's point of view, Wi-Fi uses unlicensed frequency that is free of charge. On top of that, the cost of Wi-Fi Access Points (APs), both from capital expense (CAPEX) and operational expenses (OPEX) aspects, is often lower than that of a 3GPP base station (BS) (i.e. NodeB (NB) in case of UMTS or enhanced NodeB (eNB) in case of LTE. Operators can also take advantage of already deployed APs that are already deployed in hotspots such as train stations, airports, stadiums, shopping malls, etc. Most end users are also currently used to having Wi-Fi for free at home (as home broadband subscriptions are usually flat rate) and public places.        Terminal support: Many User Equipments (UEs—the term used to refer to mobile communication devices or terminals in 3GPP), including virtually all smartphones, and other portable devices currently available in the market support Wi-Fi. In the Wi-Fi world, the term Station (STA) is used instead of UE, and as such the terms UE, STA and terminal are used interchangeably in this document.        High data rate: Under low interference conditions and assuming the user is close to the Wi-Fi AP, Wi-Fi can provide high peak data rates (for example, theoretically up to 600 Mbps for IEEE 802.11n deployments with MIMO (Multiple Input Multiple Output)).        
For a wireless operator, offering a mix of two technologies that have been standardized in isolation from each other raises the challenge of providing intelligent mechanisms for co-existence. One area that needs these intelligent mechanisms is connection management.
Although, many of today's portable wireless devices (referred to hereinafter as “user equipments” or “UEs”) support Wi-Fi in addition to one or several 3GPP cellular technologies, in many cases, however, these terminals essentially behave as two separate devices, from a radio access perspective. The 3GPP radio access network and the UE-based modems and protocols that are operating pursuant to the 3GPP specifications are generally unaware of the wireless access Wi-Fi protocols and modems that may be simultaneously operating pursuant to the 802.11 specifications. Techniques for coordinated control of these multiple radio-access technologies are needed.
A very simplified Wi-Fi architecture is illustrated in FIG. 1 and FIG. 2. On the user plane (FIG. 1), a very lean architecture is employed, where the UE/STA 20 is connected to the Wi-Fi Access Point (AP) 22, which can directly be connected to the Internet 24 and a remote application or service 26. In the control plane (FIG. 2), an Access point Controller (AC) 28 handles the management of the AP 22. One AC 28 usually handles the management of several APs 22. Security/authentication of users is handled via an Authentication, Authorization and Accounting (AAA) entity, which is shown as a RADIUS server in FIG. 2. Remote Administration Dial In User Service (RADIUS) is the most widely used network protocol for providing a centralized AAA management (and is described in RFC 2865 by The Internet Engineering Task Force (IETF), which is available from http://www.ietf.org/rfc/rfc2865.txt).
Hotspot 2.0
Different standards organizations have started to recognize the needs for an enhanced user experience for Wi-Fi access, this process being driven by 3GPP operators. An example of this is the Wi-Fi Alliance with the Hot-Spot 2.0 (HS2.0) initiative, now officially called PassPoint (“Hotspot 2.0 (Release 1) Technical Specification”, Wi-Fi Alliance® Technical Committee Hotspot 2.0 Technical Task Group, V 1.0.0). HS2.0 is primarily geared toward Wi-Fi networks. HS2.0 builds on IEEE 802.11u, and adds requirements on authentication mechanisms and auto-provisioning support.
The momentum of Hot-Spot 2.0 is due to its roaming support, its mandatory security requirements and for the level of control it provides over the terminal for network discovery and selection. Even if the current release of HS2.0 is not geared toward 3GPP interworking, 3GPP operators are trying to introduce additional traffic steering capabilities, leveraging HS2.0 802.11u mechanisms. Because of the high interest of 3GPP operators, there will be a second release of HS2.0 focusing on 3GPP interworking requirements.
HS2.0 contains the following procedures:                1 Discovery: where the terminal discovers the Wi-Fi network, and probes it for HS2.0 support, using 802.11u and HS 2.0 extensions.        2 Registration is performed by the terminal toward the Wi-Fi Hot-spot network if there is no valid subscription for that network.        3 Provisioning: Policy related to the created account is pushed toward the terminal. This only takes place when a registration takes place.        4 Access: cover the requirements and procedures to associate with a HS2.0 Wi-Fi network.        
One of the attractive aspects of HS2.0 is that it provides information for the STA that can be used to evaluate the load of the Wi-Fi network before attempting the authentication process, thereby avoid unnecessary connections to a highly loaded Wi-Fi network. The load conditions that the STA can evaluate are the following:                BSS load element—This is actually a part of the original IEEE 802.11 standard and provides information about the AP population and the current over-the-air traffic levels, as shown in FIG. 3. It is obtained either via a Beacon or a Query Response frame and is used for vendor-specific AP-selection algorithms. The element is described in detail in Chapter 8.4.2.30 of IEEE 802.11: IEEE Standard for Information technology—Telecommunications and information exchange between systems: Local and metropolitan area networks—Specific requirements—Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications (2012). The most relevant field is the “Channel Utilization” field, which states the amount of time that the AP senses the medium as busy.        WAN metrics element—This is one of the extra features that HotSpot™ 2.0 adds to the IEEE 802.11u amendment. The element, illustrated in FIG. 4, can be obtained via an access network query protocol (ANQP) query (by requesting the element “ANQP Vendor Specific list”) and it provides information about the AP's uplink/downlink wide area network (WAN) (backhaul) speed, as well as the uplink/downlink load. The element is described in detail in Chapter 4.4 of the HS2.0 specification.Wi-Fi/3GPP Integration MechanismsNo Integration—        
Most current Wi-Fi deployments are totally separate from mobile networks, and are to be seen as non-integrated (see FIG. 5). From the terminal 60 perspective, most mobile operating systems (OS) for UEs such as Android and iOS, support a simple Wi-Fi offloading mechanism, where the UEs 60 immediately switch all their PS (Packet Switched) bearers to a Wi-Fi network 62 from a fixed network 64 upon a detection of such a Wi-Fi network 62 with a certain signal level. The decision to offload to a Wi-Fi network 62 or not is referred henceforth as “access selection strategy” or “access network selection” and the aforementioned strategy of selecting Wi-Fi whenever such a network is detected is known as “Wi-Fi-if-coverage”.
There are several drawbacks of the Wi-Fi-if-coverage strategy (illustrated in FIG. 8):                Though the user/UE 60 can save previous passcodes for already accessed Wi-Fi Access Points (APs) 62, hotspot login for previously unaccessed APs usually requires user intervention, either by entering the passcode in Wi-Fi connection manager or using a web interface.        Interruptions of ongoing services can occur due to the change of IP address when the UE 60 switches to the Wi-Fi network 62. For example, a user who started a VoIP call while connected to a mobile network 64 is likely to experience call drop when arriving home and the UE switching to the Wi-Fi network 62 automatically. Some applications are smart enough to handle this and to survive the IP address change, but the majority of current applications cannot. It also places a lot of burden on application developers if they have to ensure service continuity.        No consideration of expected radio performance is made, and this can lead to a UE 60 being handed over from a high data rate mobile network link to a low data rate via the Wi-Fi link. Even though the UE's OS or some high level software is smart enough to make the offload decisions only when the signal level on the Wi-Fi network 62 is considerably better than the mobile network link, there can still be limitations on the backhaul (e.g. an xDSL connection) that the Wi-Fi AP 62 is using that may end up being the bottle neck.        No consideration of the load conditions in the mobile network 64 and Wi-Fi network 62 is made. As a result, the UE 60 might still be offloaded to a Wi-Fi AP 62 that is serving several UEs 60 while the mobile network 64 (e.g., LTE, 3G) that it was previously connected to is rather unloaded.        No consideration of the UE's mobility is made. Due to this, a fast moving UE 60 can end up being offloaded to a Wi-Fi AP 62 for a short duration, just to be handed over back to the mobile network 64. This is especially a problem in scenarios like cafes with open Wi-Fi, where a user walking by or even driving by the cafe might be affected by this. Such ‘ping pong’ between the Wi-Fi network 62 and mobile network can cause service interruptions as well as generate considerable unnecessary signalling (e.g. towards authentication servers).        
In order to combat these problems, several Wi-Fi/3GPP integration mechanisms have been proposed.
Common Authentication—
The idea behind common authentication is based on the use of automatic subscriber identity module (SIM)-based authentication in both access types. Extensible Authentication Protocol (EAP) is an authentication framework that provides support for the different authentication methods. Described by the IETF document RFC 3748 and (available from http://tools.ietf.org/html/rfc3748) later updated by RFC 5247 (available from http://tools.ietf.org/html/rfc5247), this protocol is carried directly over data-link layer (DLL) and is currently widely deployed in WLANs. The EAP framework specifies over 40 different methods for authentication, and EAP-SIM (Subscriber Identity Module) is the one that is becoming widely available in UEs and networks. FIG. 7 illustrates common authentication via EAP-SIM. A SIM 66 is shown for the UE 60 that is used in the common authentication of the user to the Wi-Fi network 62 and fixed network 64. A key benefit of common authentication is that the user doesn't necessarily have to be actively involved in the authentication process, which will increase the chances of more traffic to be steered to the Wi-Fi side, paving the way for network centric control.
User Plane Integration—
Wi-Fi user plane integration provides the mobile operator the opportunity to provide the same services, like parental control and subscription based payment methods, for the end users when connected both via 3GPP and via Wi-Fi. The solutions also include the possibility to offload parts of the user plane from the mobile core so that not all traffic needs to be brought to the mobile core network.
Different solutions are being standardized in 3GPP. Overlay solutions (S2b, S2c) are specified since 3GPP Rel-8, while integration solutions (S2a) are currently a work-in-progress (S2a, S2b, S2c indicate the 3GPP interface/reference point name towards the PDN-GW). FIG. 8 and FIG. 9 show a high level view and an architectural overview of user plane integration, respectively. With user plane integration, it is possible to access operator services like parental control, multimedia messaging (MMS), subscription payments and it is possible to still offload selected parts of traffic.
RAN Level Integration—
A further level of integration can be realized via access selection based on RAN information on both 3GPP and Wi-Fi, in addition to the common authentication and user plane integration methods discussed above.